Using acoustic camouflage is what makes Barn Owls such superb hunters, writes Ian Parsons
Barn Owls are delightful birds, a favourite of birders and non-birders alike. I get to see them fairly regularly. Sometimes I spot them perched up in an open barn, or in a tree during the day. And, when the day ends, I sometimes have them appearing ghost-like above me, their white undersides contrasting with the dark night sky, as they flash across the road as I am driving home.
Barn Owls are probably now holding their own in Britain, following years of decline, and are currently Green-listed on the UK list of birds of conservation concern, but they are a difficult bird to assess accurately, as their populations can increase and decrease dramatically in line with local vole population cycles. But, generally speaking, the Barn Owl is doing OK in Britain.
To watch a Barn Owl in the fading evening light, quartering a field in its search for small mammals, has to be one of the very best birding experiences, in my opinion. The disappearing light seems to enhance their whiteness, giving them an ethereal glow as they fly low over the ground on slow wingbeats, often alighting on a convenient fencepost to listen and look, before taking off again to continue their quest for a meal.
I enjoy watching them – doesn’t everyone? – but I also enjoy listening to them, because, as is well known, these are birds that fly on silent wings. Listening to a Barn Owl hunting is perhaps the wrong use of the word, but it adds to the overall experience because it brings it home to you just how amazing these birds are.
By listening to a Barn Owl hunting you will most probably pick up on distant traffic. Perhaps you will hear the whispering of the air through the leaves of nearby tree branches, and you will also likely hear the panicky calls of Blackbirds as they go through their evening ritual of greeting the fading daylight with alarm. But you won’t hear the Barn Owl, even when it flies close past you. Listening for this silence might sound like a strange idea, but it really brings home to you just how quiet these birds are.
Barn Owls are stealth hunters. We all know this – we know their silent flight aids them in their hunting of small mammals, animals with very acute hearing. The owls are camouflaging themselves using acoustic properties rather than colours. Now, new research has shown that this stealthiness isn’t just restricted to their flight.
Have you heard a Barn Owl land on a fence post when it is hunting? You may well have seen one do so, but have you actually heard one? Barn Owls don’t just fly silently; they can also land silently. If we go back to the aforementioned Blackbird, when they land on our garden fence, they do so with a distinctive thud. Yet if a Barn Owl landed on our garden fence (oh I wish!) it could do so silently. Blackbirds weigh on average around 90g but Barn Owls, on average, weigh around 300g. It’s three times the weight of a Blackbird, yet it is the bigger bird that can land silently.
But they don’t always do so. They choose when to do so and they do so when landing on perches while hunting. They are not just camouflaging their flight, they are also camouflaging their landings. Barn Owls will often incorporate perches when they are hunting, using them to get closer to their prey, or areas where prey are likely to be found. They use them to wait and listen for prey movement near to the perch, but of course they aren’t the only ones listening – their prey is also listening to what is going on around it.
Voles and mice rely on their hearing to keep them safe when concealed under tussocks of grass. They have very good hearing and if they hear a predator approaching, they will either dash for a burrow or they will freeze and wait for the danger to pass. The silent flight of the Barn Owl enables the bird to hunt without giving itself away to its prey, but this benefit would be lost if the bird then gave itself away by landing with a thud on a perch.
The research has shown that Barn Owls are able to consciously adjust their landing force (measured in Newtons) when perching during hunting, so that they minimise the sound they make. When they are not hunting, the birds land with far more force. The research also demonstrates that hunting success increases as the bird’s landing force decreases. In other words, they are deliberately landing in stealth mode when they are perching to listen for prey, so that they increase their chances of being successful.
Roughly speaking, the force that a Barn Owl exerts on a perch when it is landing while it is hunting is about three times the bird’s body weight, but Barn Owls can land with far more force if they want to, and they want to when they are landing on their prey. When a Barn Owl has pinpointed its prey hidden among the grasses, it drops down to catch it, and it does this by effectively landing on the grasses that are hiding the small mammal from sight. But these are not soft landings.
To successfully catch their prey, Barn Owls often have to punch their taloned feet through a mat of grassy vegetation, and that takes force. Despite their relatively light weight and their floaty flight, when these owls want to land heavily they can, exerting a formidable force of 13 times their own body weight, which is more than four times the force exerted when they are landing lightly while perching when hunting. That is a remarkable difference. Relative to body mass, Barn Owls exert the highest recorded strike force of any bird!
It is always worth putting some time in to watch Barn Owls; they are beautiful birds and even a glimpse is a rewarding sight. If you get the chance to watch them, make sure you listen, too – listen to the silence in their flight and listen to how they choose to land on a perch. They are masters at acoustic camouflage, employing stealth tactics to fly and perch as they hunt. They choose how they want to land, be that lightly or be that with maximum force. We are always learning about birds and this new research shows that there is always something else to discover about them.